In advanced electronic and photonic technologies, devices are usually fabricated from functional semiconductors, such as III-N semiconductors, III-V semiconductors, II-VI semiconductors, and Ge. The lattice constants of these functional semiconductors typically do not match the lattice constants of silicon substrates. As understood in the art, lattice constant mismatch between a substrate and an epitaxial layer on the substrate can introduce strain into the epitaxial layer, thereby preventing epitaxial growth of thicker layers without defects. Therefore, non-silicon substrates are usually employed as seeds for epitaxial growth of most functional semiconductors. However, non-Si substrates with lattice constants matching those of functional materials can be costly and therefore limit the development of non-Si electronic/photonic devices.
One method to address the high cost of non-silicon substrates is the “layer-transfer” technique, in which functional device layers are grown on lattice-matched substrates and then removed and transferred to other substrates. The remaining lattice-matched substrates can then be reused to fabricate another device layer, thereby reducing the cost. To significantly reduce manufacturing costs, it can be desirable for a layer-transfer method to have the following properties: 1) substrate reusability; 2) a minimal substrate refurbishment step after the layer release; 3) a fast release rate; and 4) precise control of release thickness.
Conventional methods to remove and transfer a device layer from a lattice-matched substrate include chemical lift-off (also referred to as epitaxial lift-off or ELO), optical lift-off (also referred to as laser lift-off or LLO, and mechanical lift-off (also referred to as controlled spalling). Unfortunately, none of these methods has the four desired properties at the same time.
The chemical lift-off technique can be used for lifting off devices layers made of III-V semiconductors from GaAs wafers. A sacrificial layer of AlAs is usually epitaxially inserted between the device layer and the substrate. Chemical lift-off technique selectively etches the sacrificial layer in a wet-chemical solution to release the device layers.
Despite its continuous development over the last three decades, chemical lift-off still has several disadvantages. For example, the release rate is slow owing to slow penetration of chemical etchant through the sacrificial layer (e.g., typically a few days to release a single 8-inch wafer). Second, etching residues tend to become surface contamination after release. Third, chemical lift-off has limited reusability owing to the chemical mechanical planarization (CMP) performed after release to recover the roughened substrate surface into an epi-ready surface. Fourth, it can be challenging to handle released epilayers in the chemical solution.
The optical lift-off technique usually uses a high-power laser to irradiate the back of the lattice-matched substrate (e.g., a transparent sapphire or SiC substrate) and selectively heat the device-substrate interface, causing decomposition of the interface and release of the device layer (e.g., III-N film). This technique can reduce the cost of manufacturing III-N-based light emitting diodes (LEDs) and address the problem of heat accumulation from the device by transferring released III-Ns to a substrate that has high thermal conductivity.
However, optical lift-off has its own limitations. First, because the molten III-N/substrate interface can make the substrate rough, a reconditioning step is usually carried out before reuse, thereby reducing the reusability to less than five times. Second, local pressurization at the interface caused by high-power thermal irradiation can induce cracks or dislocations. Third, the laser scanning speed can be too slow to permit high-throughput.
Controlled spalling can have a higher throughput than optical lift-off. In this technique, high-stress films (also referred to as “stressors”) are deposited on the epitaxial film, inducing fracture below the epilayers and resulting in the separation of active materials from the substrate. When sufficient tensile stress is applied to the interface, a KII shear mode can initiate a crack and a KI opening mode can allow the propagation of the crack parallel to the interface between the epilayer and the substrate. By controlling the internal stress and thickness of the stressor, strain energy sufficient to reach the critical KI can be provided, leading to fracture of the film/substrate interface. Because the exfoliation occurs via crack propagation, the spalling process can cause rapid release of films.
However, controlled spalling is not mature enough to be used for commercial manufacturing for at least the following reasons. First, because crack propagation generally occurs through cleavage planes that are not always aligned normal to the surface, the surface may need polishing for reuse. Second, a thick stressor is usually used to provide enough energy to separate strong covalent bonds, particularly when working with high Young's modulus materials like III-N semiconductors. Third, the internal stress of the stressor may only be controlled in a narrow range, which constrains the achievable thickness of the resulting spalled film. For example, because the maximum internal stress in a typical Ni stressor is about 1 GPa, the critical Ni thickness under 1 GPa tensile stress to initiate spalling of a GaAs film is about 1.5 μm, which can induce spalling of the GaAs film itself if the GaAs is about 10 μm thick. Therefore, when using a Ni stressor it can be challenging to make a GaAs film less than 10 μm thick, but typically most devices use films that are much thinner.